Carburized low silicon steel article and process

ABSTRACT

A process for forming a carburized steel article includes carburizing a steel material containing not more than 0.10% silicon and less than 1.1% chromium to form an austenitic surface matrix having a high density of carbides dispersed therein. After quenching, the carburized steel article is characterized by an outer surface having a high ratio of carbides and is substantially free of intergranular oxides. As a result of preventing undesirable surface oxide formations and simultaneously providing a beneficial surface carbide structure, the bending fatigue strength, wear properties, and contact fatigue strength of articles such as gears, shafts, bearings and couplings are greatly enhanced.

Cross Reference to Related Application

This application is a continuation-in-part of application Ser. No.135,775, filed Dec. 21, 1987, and now abandoned. This application wasfiled through the PCT Office as International Application No.PCT/US88/04470 on Dec. 14, 1988.

TECHNICAL FIELD

This invention relates generally to metal heat treatment and moreparticularly to case hardening with a gas containing carbon and aresulting article.

BACKGROUND ART

Carburizing is an effective method of increasing the surface hardness oflow carbon, unalloyed, or low carbon, low alloy steels. Typically, steelarticles are placed in an atmosphere containing carbon in an amountgreater than the base carbon content of the steel and heated to atemperature above the austenite transformation temperature of the steel.After the desired amount of carbon has been diffused into the article,hardness is induced by quenching.

Gas carburizing is efficient, controllable, and one of the most widelyused methods of generating a carbonaceous atmosphere for carburizing.However, most commonly used gas mixtures typically contain small amountsof oxygen which tend to form surface oxides with one or more of thesteel elements which have a strong affinity for oxygen, such as silicon,chromium and manganese. Oxides that form along grain boundaries, i.e.,intergranularly, extend inwardly from the surface and have a harmfuleffect on the mechanical properties of the carburized article.

Conventional carburizing processes typically avoid the formation of casecarbides, and generally produce an essentially carbide free martensiticstructure. Even when case carbides are involuntarily formed,characteristically there is a thin surface layer void of carbides. Thisis due to the oxidation of carbide forming elements in the surfacelayer.

The detrimental effects of oxidation during gas carburizing have beenknown for a long period of time. Heretofore, when designing a carburizedarticle, it has been necessary to consider the reduction in bendingfatigue strength attributable to intergranular surface oxides. To avoidreduction in bending fatigue properties, it has been necessary tophysically remove intergranular surface oxides formed during carburizingby machining or grinding, or prevent such surface formations by removingoxygen compounds from the carburizing media. These alternatives arecostly. A 1978 article authored by Ruth Chatterjee-Fischer, "InternalOxidation During Carburizing and Heat Treating," MetallurgicalTransactions A, published by American Society for Metals and theMetallurgical Society of AIME, Vol. 9A, Nov. 1978, pp. 1553-1560,reported that, when employing conventional carburizing processes, thepresence of silicon in the parent metal is a prime contributor to theformation of oxides at the surface of the article. However, whileidentifying that lowering silicon is a solution to the problem ofsurface oxidation, the paper fails to recognize any interrelationshipbetween low silicon amounts and the enhanced formation of surfacecarbides by nonconventional carburizing methods.

A low silicon carburizing steel, developed by Sanyo Special Steel Co.,Ltd. is described in Japanese Patent Publication No. 57-23741. The Sanyoreference teaches that low silicon, i.e., 0.06% to 0.12%, when combinedwith relatively high carbon and chromium amounts will accelerate carbondiffusion and thereby reduce carburizing time. This reference limits theamount of chromium in the steel composition to intentionally avoid theformation of case carbides. Furthermore, this reference fails to linkthe influence of a low silicon composition with the formation of surfaceoxides and carbides during carburizing. The Sanyo low silicon steelcomposition described in the above publication is no longer incommercial production.

Another low silicon carburizing steel intended for use in applicationswherein the formation of case carbides is purposefully avoided, wasdeveloped by Kobe Steel, Ltd. Kobe's composition is described inJapanese Patent Publication No. 61-253346 and, as described therein, wasdeveloped for use in a gear that is heat treated after carburizing toimprove the hardness of the root areas of gear teeth. To assurefavorable surface hardenability after carburizing, Kobe intentionallylimits the amount of carbide forming elements in the steel compositionto prohibit the formation of surface carbides during carburizing. Also,the amounts of silicon, manganese and chromium in the composition arerestricted to prevent the formation of granular oxides which lower theheat treatability of the carburized surface layer. This reference alsodoes not disclose any interrelationship between low silicon amounts andthe beneficial formation of surface carbides.

A low silicon steel composition intended for carburized articles inwhich near surface carbides are formed is disclosed by Daido SpecialSteel Co. in Japanese Patent Publication No. 61-104065. Daido, like theabove references, recognizes that restricting the amount of silicon inthe steel composition is beneficial for reducing intergranular oxides.However, this reference also teaches that more than 1.2% chromium isessential to form carbides at or near within a zone extending inwardlyfrom the surface to a depth of 0.1 mm below the surface. This referencealso fails to recognize that in addition to its effect on oxidation,silicon effects surface carbide formation and, by limiting the amount ofsilicon in a carburizing steel as taught by the present invention,surface carbides may be easily formed with significantly smaller amountsof chromium.

Conventional gas carburizing processes, as discussed above, generallyattempt to prevent the formation of case carbides. A nonconventionalcarburizing process for intentionally forming carbides in the case isdescribed in Canadian Patent 610,554, "Carburization of Ferrous Alloys,"issued Dec. 13, 1960, to Orville E. Cullen. Cullen teaches a method forcarburizing low alloy steel by repeatedly carburizing and rapidlycooling the steel article. However, Cullen's method is very lengthy,requiring on the order of about 42 hours to complete, and is thereforequite expensive. Further, Cullen's process does not suggest a solutionto the problem of surface oxide formation.

More recently, a two stage carburizing process was described in U.S.Pat. No. 4,202,710, issued May 13, 1980, to Takeshi Naito, et al. Thatprocess forms spheroidal carbides within a region between 0.1 mm and 0.4mm below the case surface, but fails to provide a high density ofcarbides on the outer surface of the carburized case. As a result,articles formed by this teaching must initially wear, or be machined,down to the carbide rich zone beginning 0.1 mm below the surface beforethe enhanced wear and contact fatigue properties of the carbidemicrostructure, such as pitting and spalling resistance, can beadvantageously utilized. Also, like Cullen, this process fails to offera solution to the problem of surface oxide formation.

The present invention is directed to overcoming the problems set forthabove. It is desirable to have a carburized steel article which isessentially free of surface intergranular oxides and which has a highratio of carbides on the surface without requiring any removal ofmaterial from the carburized surface. It is also desirable to provide aprocess which will form this article in a short treatment time, and iseconomical and controllable.

DISCLOSURE OF THE INVENTION

In accordance with one aspect of the present invention, a process forforming a carburized steel article includes carburizing an article madeof a steel having no more than 0.10% silicon and less than 1.1% chromiumat a temperature sufficient to form, on a surface of the article,austenite having a high density of carbides dispersed therein. Aftercarburizing, the article is quenched to transform the carburized surfacemicrostructure to martensite, retained austenite and carbides, wherebythe article has a carburized surface substantially free of intergranularoxides and has carbides comprising at least 20% of the carburizedsurface.

Other features of the process of forming a carburized steel articleinclude carburizing in a first stage to form about 75% to 95% of thedesired case depth, which may be advantageously followed by a secondstage wherein the high density of surface carbides are formed.

In another aspect of the present invention, there is provided acarburized low silicon steel article characterized by having not morethan 0.10% silicon and less than 1.1% chromium, by having an outersurface substantially free of intergranular oxides, and by having a highpercentage of carbides on the outer surface. Preferably the carbides onthe outer surface comprise at least 20% thereof.

Other features of the carburized low silicon article include acomposition, by weight, of 0.08% to 0.35% carbon, 0.3% to 1.7%manganese, 0.2% to 2.5% carbide forming elements including saidchromium, less than 6% additional hardenability agents, less than 1%grain refining elements, not more than 0.10% silicon, not more than0.15% copper, and iron and trace elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photomicrograph, at 750X, showing an unetched section of acarburized low silicon steel article containing less than 0.10% siliconaccording to the present invention.

FIG. 2 is a photomicrograph, at 750X, of an etched section of thearticle shown in FIG. 1.

FIG. 3 is a photomicrograph, at 750X, showing an unetched section of anarticle carburized with the article in FIG. 1, but formed of a typicalcarburizing grade low alloy steel containing 0.26% silicon.

FIG. 4 is a photomicrograph, at 750X, of an etched section of thearticle shown FIG. 3.

FIG. 5 is a photomicrograph, at 500X, showing an etched section of anarticle formed of a typical carburizing grade low alloy steel containing0.26% silicon and carburized by a conventional carburizing process.

FIG. 6 is a graph showing the relationship between silicon content inthe steel composition, and the amount of oxides and carbides formed onthe surface of an article carburized according to the present invention.

FIG. 7 is a graph illustrating the time and temperature relationship ofthe carburizing process embodying the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A process for forming a steel article includes the steps of selecting alow silicon steel, shaping the steel, carburizing, controllably coolingto the hardening temperature, and quenching. As used herein the term"low silicon steel" means a steel material containing from 0% silicon tono more than 0.10% silicon, and whose composition in all other respectsis recognized as generally suitable for carburizing. Restricting, oreliminating, the amount of silicon in the steel not only represses theformation of silicon oxides but also inhibits the oxidation of chromiumand manganese on the surface of the carburized article. Furthermore,when low silicon steel articles are processed in accordance with thepresent invention, the formation of a high ratio of carbides on thesurface is assured. Accordingly, selection of an appropriate steelcomposition is an important step.

Commercially produced low alloy carburizing grade steels typically havea composition, by weight, within the following ranges:

    ______________________________________                                        Carbon             0.08%-0.35%                                                Manganese          0.3%-1.7%                                                  Carbide forming    0.2%-2.5%                                                  elements such as                                                              Chromium, Molybdenum                                                          or Vanadium (total)                                                           Additional         0.0%-6%                                                    Hardenability agents                                                          Silicon            0.15%-0.35%                                                Grain refining elements                                                                          0.0%-1.0%                                                  Iron and residual  Balance                                                    elements                                                                      ______________________________________                                    

The above steel composition, if silicon is deleted or restricted to0.10% or less, is suitable for the formation of carburized articlesaccording to the present invention.

The accepted steel industry standard allowable residual amount of copperis steel compositions is 0.35%. It has been found that low siliconsteels having copper toward the upper limit of such amount are sometimesinconsistent in the formation of carbides on the carburized surface. Itis theorized that copper may inhibit the formation of surface carbides.Accordingly it is desirable, though apparently not essential in allcases, to limit the amount of copper in the composition to not more than0.15%.

More specifically, a low alloy carburizing grade steel, such as one ofthe SAE 4110 to 4130 series, all modified particularly to limit siliconto no more than 0.10%, are particularly suited for the formation ofcarburized articles such as gears, bearings and shafts. Therefore,commercial low alloy carburizing grade steels containing relativelysmall but essential amounts of carbide forming elements such aschromium, molybdenum or vanadium, may be modified to be a low siliconsteel composition. Additional hardenability agents may be included butshould be limited to amounts less than about 6%. Grain refining elementsin amounts less than 1% are often added to promote fine grain size. Asnoted above, it is also desirable to limit residual or trace amounts ofcopper to no more than 0.15% to further enhance surface carbideformation. Deleterious elements such as phosphorus and sulfur, oftenpresent in trace amounts, are preferably limited to no more than 0.05%and 0.08% respectively.

Articles having any of the above described compositions are shaped to apredetermined form by machining from rolled steel, by casting orforging, by consolidating steel powder, or by a combination of formingoperations.

After shaping, the article is placed into a carburizing furnace andheated to a temperature sufficient to diffuse carbon from the furnaceatmosphere into the article and to form a high density of carbides onthe surface of the article. Preferably, the carburizing operation iscarried out in two stages. In the first carburizing stage, the carbonpotential of the gas atmosphere in the furnace is maintained at a levelabout equal to the saturation limit of carbon in austenite. Thesaturation level of carbon in austenite, generally designated as "A_(cm)", is temperature dependent. The shaped article is preferably placeddirectly into a continuous type furnace having a temperature within thecarburizing chamber of from about 1550° F. (843°C.) to about 1850° F.(1010° C.) and an atmosphere about equal to the A_(cm) corresponding thespecific steel composition of the article.

As shown graphically on FIG. 7, the first stage of the carburizingprocess includes holding the article in the carburizing chamber for aperiod of time sufficient to develop about 75% to 95% of the desiredfinal case depth. As used herein, "case depth" is the distance below thesurface where the carbon content is at 0.40%. The time required toachieve the desired case depth of the article is dependent on a numberof parameters, such as the chemical composition of the article, andcomposition and temperature of the gas atmosphere. Hence the length oftime that the article is held in the carburizing chamber in this firststage may vary from about 2 to about 25 hours. When the article has thelow silicon composition described above, the furnace atmosphere ismaintained as close as possible to the A_(cm) of the particular steel,and the carburizing temperature is maintained at about 1700° F. (927°C.), an initial case depth of about 1.0 mm can be achieved by holdingthe article in the furnace for about 4 to 5 hours. After completion ofthe second stage carburizing operation, described below, the final casedepth is about 1.1 mm.

After the first stage carburizing operation, the article is cooled,preferably by gas quenching, to a temperature below that at whichbainite and pearlite begin to form. This temperature is commonlydesignated as "A_(rl) " and is represented by the dashed horizontal lineon FIG. 7. It is desirable to cool the surface of the article rapidly inthis operation to suppress network carbide formation. This results inthe formation of a supersaturated, metastable, solid solution of ironand carbon. If excessively slow cooling rates are used, carbides willform at grain boundaries, and not provide effective intragranular nucleifor the formation of carbides.

After cooling the article to a temperature sufficiently below A_(rl) toassure the substantially complete transformation to bainite or pearlite,the article is quickly reheated by placing it into a preheatedcarburizing furnace. This causes a homogenous carbide precipitation fromthe metastable solid solution, and results in a dispersion of carbidesin the case microstructure.

During the second stage carburizing operation, the carbon potential ofthe furnace atmosphere is maintained at a level above the A_(cm), i.e.,the carbon potential of the atmosphere is above the saturation limit ofcarbon in austenite of the article. For example, an atmosphere having acarbon potential of about 1.5% to 2% can be provided by a gas having acomposition of 0.055% CO₂, 1% CH₄, 20% CO, 40% H₂ and the balance N₂.Typically, the temperature of the second stage carburizing chamber ismaintained between about 1550° F. (843° C.) and 1850° F. (1010° C.), andpreferably at about 1700° F. (927° C.). In the second stage it is onlynecessary to reaustenitize the case of the article in a supersaturatedgas environment, and thereby increase the surface carbon to a levelabove 1.5%. The article need be held for only about 15 to 60 minutes at1700° F. (927° C.) to achieve the desired case carbon content. Althoughlower temperatures may be used, it will require holding the article inthe furnace for a corresponding longer time. Also, it has been foundthat the size and volume fraction of the surface carbides present in thecarburized article can be controlled by selection of the second stagecarburizing temperature and the rate of cooling in the subsequentcooling step described below. For example, higher second stagecarburizing temperatures tend to produce more carbides, i.e., a highervolume fraction of larger carbides, on the surface of the article.

Upon completion of the second stage carburizing operation an austeniticmicrostructure having a high density of well dispersed surface carbideshas been formed. The article, preferably while still in the furnace, isthen controllably cooled to the hardening temperature of the steel core.By way of example, the hardening temperature of SAE 4118 steel having alow silicon composition, is about 1540° F. (838° C.). During the coolingoperation, the previously formed carbides increase in size and volumefraction. To achieve this, the cooling rate of the article from thecarburizing to hardening temperature is carefully controlled.Advantageously, the cooling rate is about 1° F. (0.6° C.) to 20° F. (11°C.) per minute.

After cooling the surface of the article to the hardening temperature,it is desirable to maintain the article at the hardening temperature fora length of time sufficient to permit the temperature in the core of thearticle to cool to the hardening temperature. This process, known asequalizing, is identified in the process diagram shown in FIG. 7.Although desirable to minimize distortion, equalizing is not essentialto obtain the desired surface microstructure. Depending on the article'smass and geometry, equalizing typically takes about 5 to 60 minutes tocomplete.

A carbonaceous gas atmosphere is advantageously maintained about thearticle during the preceding cooling and equalizing operations toprevent carbon depletion at the surface of the article. Both cooling andequalizing may be carried out in the second stage carburizing furnace,or in an interconnecting chamber, so that the same gas atmosphere usedin carburizing may be simply cooled and circulated about the article.

After equalizing, the article is preferably directly quenched from thehardening temperature at a rate sufficiently rapid to transform thesurface microstructure to martensite, retained austenite and carbides.For this, an oil medium is used with a quench rate high enough to assurethe desired surface transformation.

If it is not convenient to directly quench the article immediately afterequalizing, alternative steps represented by the dashed line in FIG. 7have been developed. In one alternative, the article may be cooled andthen reheated to the hardening temperature of the steel, equalized andquenched. In another alternative, if the mass and geometry of thearticle are sufficiently small, and the controlled cooling ratesufficiently slow, equalizing may not be required. If these conditionsare present, the article may be quenched directly after the controlledslow cooling step.

Shafts and gears are exemplary of articles subjected to high bendingloads, surface wear and contact fatigue. Samples of these articles havebeen successfully formed in accordance with the above described process.The total time required to complete the carburizing process, includingthe final quench, was typically only about 7 hrs. Thus, the describedprocess not only forms articles having improved metallurgical surfacecharacteristics without requiring further finishing of the carburizedsurface, but it is also much faster than prior art methods for formingdesirable surface carbide structures.

Photomicrographs of a representative sample of an article embodying thepresent invention are shown in FIGS. 1 and 2. Advantages of the presentinvention are emphasized by contrasting it with FIGS. 3 to 5 which arephotomicrographs of representative samples of articles formeddifferently as described below. FIGS. 1 and 3, are photomicrographs ofpolished and unetched samples in which intergranular oxides areidentified. FIGS. 2 and 4 are the same samples as FIGS. 1 and 3, butetched with a conventional 1% nital solution to better define thecarbides in the microstructure. FIG. 5 is a sample of an article given aconventional heat treat, and polished and etched with 1% nital solution.In the photomicrographs, intergranular oxides appear as elongated blackareas in the unetched samples. Typical intergranular oxide formationsare identified by the reference numeral 10. Carbide structures appear inthe photomicrographs of the etched samples as white areas, typical onesof which are identified by the reference numeral 12.

EXAMPLE 1

A representative sample of an article formed according to one embodimentof the present invention has a microstructure as shown in section, at750X, in FIGS. 1 and 2. The sample was cut from a modified wrought SAE4118 steel having the following specific composition:

    ______________________________________                                        Carbon                0.23%                                                   Manganese             0.99%                                                   Phosphorus            0.009%                                                  Sulfur                0.010%                                                  Silicon               0.04%                                                   Chromium              0.40%                                                   Molybdenum            0.13%                                                   Aluminum              0.029%                                                  Copper                0.03%                                                   Iron and residual     Balance                                                 elements                                                                      ______________________________________                                    

The sample was placed in a carburizing furnace having a conventionalendothermic gas carburizing atmosphere comprising about 0.07% CO₂, 0.6%CH₄, 20% CO, 40% H, and the balance N₂. The furnace was preheated to atemperature of about 1700° F. (927° C.). The carbon potential of theatmosphere was maintained as close as the furnace control systempermitted to the A_(cm), the carbon in austenite solubility limit of theabove described steel composition. The article was held in the furnace,at the same temperature and atmosphere for 4 1/2 hrs, and then gasquenched to 200° F. (93° C.). After cooling subsequent to the firstcarburizing operation, the article was placed in a carburizing furnace,preheated to about 1700° F. (927° C.), and the gas atmosphere controlledso that the carbon potential of the gas was greater than the saturationlimit of carbon in the steel material's austenite phase. The article washeld in the carbon rich atmosphere, at the same temperature, for about30 minutes, and then slowly cooled over a period of about 60 minutes,i.e., at a rate of about 2.7° F./minute (1.5° C./minute) to 1540° F.(838° C.), the hardening temperature of the steel. The article was heldat 1540° F. (838° C.) for about 40 minutes to allow the temperature ofinterior portions of the article to equalize to the hardeningtemperature. After equalizing, the article was directly quenched in anoil medium.

As may be seen in the photo of a micrographic sample of the article inFIG. 2, the article has a high volume ratio of carbides 12 formed on theas-carburized surface. Direct measurement of the carbides on the surfaceimmediately adjacent the band of black bedding material observable alongthe very top of the photomicrograph shows that carbides comprise about50% of the article's surface area. Furthermore, as best shown in theunetched section shown in FIG. 1, the carbide rich area formed on thesurface of the article is essentially free of intergranular oxides.

EXAMPLE 2

A second article, a micrographic sample of which is shown in FIGS. 3 and4, was formed from modified wrought SAE 4118 steel having a siliconcontent within the typical range of silicon for commercially producedheats of similar material. Specifically, the article in this example hasthe following composition:

    ______________________________________                                        Carbon                0.22%                                                   Manganese             0.76%                                                   Phosphorus            0.009%                                                  Sulfur                0.016%                                                  Silicon               0.26%                                                   Chromium              0.78%                                                   Molybdenum            0.24%                                                   Aluminum              0.033%                                                  Copper                0.12%                                                   Iron and residual     Balance                                                 elements                                                                      ______________________________________                                    

The material in this example has a silicon content of 0.26%,representative of silicon amounts normally present in conventionalcarburizing grade steels having a similar composition. The articleformed of the above material was heat treated simultaneously with thearticle described in Example 1, i.e., according to the process of thepresent invention.

FIG. 4 illustrates the influence of increased amounts of silicon onsurface carbide formation. There are virtually no carbides 12 on thesurface of this sample. Furthermore, the carbide free surface layer iscommensurate with the depth of the intergranular oxides 10. Therefore,even when employing a case carbide forming, nonconventional carburizingprocess, it is necessary to limit the amount of silicon in the basesteel to assure the formation of significant carbide structures on thesurface of the carburized article. Further, as best seen in FIG. 3,intergranular oxides 10 are present in significantly greater quantitythan in the low silicon material of FIG. 1. The ratio of intergranularoxides to length of article surface shown in the field of view is about0.50:1. That is, when measured along their major axes, the length of theintergranular oxides visible in FIG. 3 total about 50% of the length ofthe surface in the same field of view.

EXAMPLE 3

A sample of an article having a modified SAE 4118H composition with0.26% silicon, and given a conventional single stage carburizingtreatment, is shown in FIG. 5. The article has the followingcomposition:

    ______________________________________                                        Carbon                0.21%                                                   Manganese             0.93%                                                   Phosphorus            0.011%                                                  Sulfur                0.013%                                                  Silicon               0.26%                                                   Chromium              0.95%                                                   Molybdenum            0.34%                                                   Aluminum              0.032%                                                  Copper                0.06%                                                   Iron and residual     Balance                                                 elements                                                                      ______________________________________                                    

This example illustrates the metallurgical surface characteristics ofconventionally treated standard carburizing grade steel in which casecarbides were formed. In particular, there is a typical carbide depletedzone extending about 0.02 mm below the surface of the article. Withinthis zone there are a significant number of oxide formations, bothintergranular and intragranular. As is well known, intergranular oxides10 have a deleterious affect on bending fatigue properties. Furthermore,because there are no carbides on the surface, about 0.02 mm of thearticle's outer surface must be worn away before the benefit of thecarbide structures can be used advantageously.

As can be seen from the above examples there are several features of thedescribed embodiment. First, the steel material from which an article isformed must contain no more than 0.10% silicon. Limiting the siliconcontent to this amount not only inhibits the formation of intergranularoxides, but also promotes and enhances the formation of desirablecarbide structures on the surface. Secondly, a high ratio of carbidesare formed on the outer surface of the article, (i.e., the as-carburizedsurface without any material removal therefrom), and not at some finitedepth, e.g. 0.1 mm or more, below the surface. To accomplish this mostadvantageously, the article is carburized in the manner described toassure the formation of a high density of carbides well dispersed inaustenite on the article surface prior to growth by controlled cooling.As seen from Examples 2 and 3, if either of these two features aremissing, the carburized article will not have an outer surface which isessentially free of intergranular oxides and on which a high volumeratio of carbides are formed.

EXAMPLE 4

A number of samples having similar base compositions except for siliconamounts, and in two examples, chromium, were prepared and heat treatedby the carburizing process described in Example 1, above. The sampleswere then cut, polished, and analyzed for the presence of intergranularoxides and surface carbides. Specifically, nine samples havingcompositions as listed below were tested:

    __________________________________________________________________________                                  *SURFACE RATINGS*                                   COMPOSITION               Intgr.                                          Sample                                                                            (Balance Fe and residual elements)                                                                      Oxides                                                                              Carbides                                  No. % C                                                                              % Mn                                                                              % Cr                                                                              % Mo                                                                              % Al                                                                              % Cu                                                                              % Si                                                                             % ratio                                                                             % ratio                                   __________________________________________________________________________    a   0.22                                                                             0.67                                                                              0.89                                                                              0.19                                                                              0.003                                                                             0.02                                                                              0.01                                                                               0%   51%                                      b   .23                                                                              .98 .43 .13 .040                                                                              .03 .04                                                                               1    45                                        c   .23                                                                              .89 .45 .21 .033                                                                              .03 .06                                                                               6    42                                        d   .23                                                                              .84 .45 .20 .029                                                                              .03 .09                                                                               8    27                                        e   .23                                                                              .83 .44 .20 .027                                                                              .03 .11                                                                              13    10                                        f   .22                                                                              .80 .44 .20 .030                                                                              .03 .12                                                                              17    17                                        g   .22                                                                              .78 .44 .20 .025                                                                              .03 .14                                                                              26    12                                        h   .23                                                                              .77 .45 .20 .023                                                                              .03 .18                                                                              25     8                                        i   .22                                                                              .76 .78 .24 .033                                                                              .12 .26                                                                              37     0                                        __________________________________________________________________________

The total length of intergranular oxides, measured along the major axesof the oxides in three equally-sized representative fields in a sectionof each sample, were averaged and expressed as a % ratio to the lengthof the article's outer surface in the field of view. Similarly, thepercent of linear surface at which carbides are present was measured inthree equally-sized representative fields in a section of each sample,averaged, and expressed as a % ratio to the length of the articlesurface in the representative field of view.

The results of the above measurements are shown in a graphical form inFIG. 6. The % ratio of oxides for each of the above samples isrepresented on the graph by a small circular symbol. The scale for %silicon is shown along the base of the graph, and the scale for % ratioof oxides is shown along the right vertical side. As shown by the solidline representing a reasonable fit for the plotted data points, theamount of intergranular oxides increases with increasing silicon contentin the samples. The data indicates that for these particular samples,the silicon content may be as high as 0.11% or 0.12% and still yieldless than about a 15% ratio of intergranular oxides with respect to arepresentative surface length.

In similar fashion, the % ratio of carbides for each of the abovesamples is represented on the graph by a small triangular-shaped symbol,and plotted according to the scale shown along the left vertical side ofthe graph. A line, representing a reasonable fit of the individual datapoints, readily shows that the % ratio of carbides with the respect tothe surface generally decreases with increases in silicon in thesamples. Sample e appears to have a lower carbide ratio than eithersample d or f, respectively representing lower and higher siliconamounts on each side of sample e. For this reason, although theintegrated data suggests that, for the above representative group ofsamples, a silicon content of about 0.11% or 0.12% will provide adesirable carbide level of about 20%, a preferred limit of about 0.10%silicon will more consistently assure that the carbide to surface ratioin the carburized article will be above 20%.

INDUSTRIAL APPLICABILITY

Articles formed according to the above are particularly useful as gears,couplings, shafts, bearings, and similar articles subjected to acombination of high bending loads, surface wear and contact fatigue.

It has been found that low silicon steel with restricted residualamounts of copper is priced about the same as conventional carburizinggrade alloy steels. Accordingly, since the process is more economicalthan many conventional processes, it is felt that the process may havewide applicability and the resulting articles, which do not require anyfurther finishing of the carburized surface, may also be widely usedwhere carburized articles are required.

It is now deemed apparent that there has been described a carburizingprocess for low silicon steel material to form a high ratio of carbideson the outer surface of an article substantially free of intergranularsurface oxides. It is known that intergranular surface oxides reduce thebending fatigue strength of articles. It is also known that high densitycarbide structures have high hardness and contribute to increased wearand contact fatigue life. As a result of preventing undesirable surfaceoxide formations and simultaneously providing beneficial surface carbidestructures on the as-carburized surface, the bending fatigue strength,wear properties, and contact fatigue properties of articles formed asdescribed are greatly enhanced.

Other aspects, features and advantages of the present invention can beobtained from a study of this disclosure together with the appendedclaims.

We claim:
 1. A process for forming a steel article having a carburizedsurface substantially free of intergranular oxides and having carbideson said carburized surface comprising at least 20% thereof, notrequiring any further finishing, including the steps of:selecting asteel material having, by weight percent, from about 0.08% to about0.35% carbon, from about 0.3% to about 1.7% manganese, not more than0.10% silicon, not more than 0.15% copper, less than 1.1% chromium, fromabout 0.2% to about 2.5% carbide forming elements including saidchromium, less than 6% additional hardenability agents, less than 1%grain refining elements, and the balance iron and trace impurities;shaping the steel material to form an article; carburizing said articleat a temperature and for a period of time sufficient to form a surfacecomprising austenite and a high density of carbides dispersed in saidaustenite; quenching said carburized article to transform the surfaceinto a microstructure of martensite, retained austenite and carbides;and controlling the steps of carburizing and quenching so that thecarbides comprise at least 20% of said surface, and the surface issubstantially free of intergranular oxides, thereby producing acarburized steel article not requiring any further finishing of thecarburized surface.
 2. A process for forming a carburized steel article,as set forth in claim 1, wherein the step of quenching the carburizedarticle from the hardening temperature of the steel material includesquenching in an oil medium.
 3. A process for forming a carburized steelarticle, as set forth in claim 1, including prior to quenching, the stepof controllably cooling at least the surface of the carburized articlefrom the carburizing temperature to the hardening temperature of saidsteel material at a rate sufficient to assure that the carbides increasein size and form a higher surface fraction of carbides.
 4. A process forforming a carburized steel article, as set forth in claim 3, wherein thestep of cooling said carburized article to the hardening temperatureincludes reducing the temperature of the furnace atmosphere at a rate offrom about 1° F./minute (0.6° C./minute) to about 20° F./minute (11°C./minute).
 5. A process for forming a carburized steel article, as setforth in claim 3, including after cooling to the hardening temperature,the step of maintaining said carburized article at the hardeningtemperature of the steel material for a time sufficient to permit thecore temperature of the article to equalize at said hardeningtemperature.
 6. A process for forming a carburized steel article, as setforth in claim 5, wherein the step of maintaining said carburizedarticle at the hardening temperature of the steel material includesmaintaining the article at a temperature of about 1540° F. (838° C.) forfrom about 5 minutes to about 60 minutes.
 7. A process for forming acarburized steel article, as set forth in claim 1, wherein the step ofcarburizing said article includes a first stage in which the article isplaced in a preheated furnace having a temperature of from about 1550°F. (843° C.) to about 1850° F. (1010° C.) and an atmosphere in which thecarbon content is maintained essentially in equilibrium with austenitesaturated with carbon at the furnace temperature, and held in saidfurnace for a period of time sufficient to form from about 75% to about95% of the final case depth.
 8. A process for forming a carburized steelarticle, as set forth in claim 7, wherein during said first stage thefurnace temperature is maintained at about 1700° F. (927° C.) and saidarticle is held in said furnace from about 4 to about 5 hours.
 9. Aprocess for forming a carburized steel article, as set forth in claim 7,wherein the step of carburizing includes gas quenching said articleafter the first stage carburizing, said quenching being at a ratesufficient to suppress carbide nucleation in the carbuized case andmaintaining said rate until the case temperature is below thetemperature at which the transformation of austenite is essentiallycomplete.
 10. A process for forming a carburized steel article, as setforth in claim 9, wherein the step of carburizing the article includes asecond stage in which the article is placed in a preheated furnacehaving a temperature of from about 1550° F. (843° C.) to about 1850° F.(1010° C.) and an atmosphere in which the carbon content is maintainedat a level greater than the saturation limit of carbon in austenite atthe furnace temperature, and held in said furnace for a period of timesufficient to transform the surface to an essentially austeniticmicrostructure having a high density of surface carbides dispersed insaid austenite.
 11. A process for forming a carburized steel article, asset forth in claim 10, wherein during the second stage, the furnacetemperature is maintained at about 1700° F. (927° C.) and the article isheld in said furnace for from about 15 minutes to about 60 minutes. 12.A process of carburizing a low silicon steel article, to provide anarticle having an as-carburized surface which is substantially free ofintergranular oxides and with carbides on at least 20% thereof,including the steps of:carburizing an article made of a steel having nomore than 0.10% silicon by weight and less than 1.1% chromium by weightat a temperature and for a time sufficient to form, on a surface of thearticle, austenite having a high density of carbides dispersed thereinthereby forming a carburized surface; and thereafter controllablyquenching the article to transform the carburized surface microstructureto martensite, retained austenite and carbides; so that the article hasan as-carburized surface substantially free of intergranular oxides andhas carbides comprising at least 20% of said carburized surface, therebyproducing a carburized low silicon steel article not requiring anyfurther finishing of the carburized surface.
 13. A carburized lowsilicon steel article characterized by having not more than 0.1% siliconand less than 1.1% chromium, by having a carburized surface that,without any material removed therefrom, is substantially free ofdetrimental intergranular oxides, and has carbides on said carburizedsurface which comprise at least 20% of the area of the carburizedsurface, and does not require further finishing.
 14. A carburized lowsilicon steel article, as set forth in claim 13 which is virtually freeof any intergranular oxides at the entire carburized surface.
 15. Acarburized low silicon steel article, as set forth in claim 13, whereinthe total length of the intergranular oxides in a representativecross-section of the article is not more than about 15% of the length ofthe carburized surface of the article in said cross-section.
 16. Acarburized low silicon steel article, as set forth in claim 13, whereinthe article's composition, by weight, includes about 0.08% to 0.35%carbon and about 0.2% to 2.5% carbide forming elements including saidchromium.
 17. A carburized low silicon steel article, as set forth inclaim 13, wherein said article has a composition consisting essentiallyof by weight, of 0.08% to 0.35% carbon, 0.3% to 1.7% manganese, not morethan 0.10% silicon, less than 1.1% chromium, 0.2% to 2.5% carbideforming elements including said chromium, less than 6% additionalhardenability agents, less than 1% grain refining elements, not morethan 0.15% copper, and the balance iron and trace impurities.
 18. Acarburized steel article, as set forth in claim 17, wherein carbon ispresent in an amount from 0.18% to 0.24%, manganese is present in anamount from 0.8% to 1.1%, chromium is present in an amount from 0.4% to1.1%, molybdenum is present in an amount from 0.1% to 0.5%, silicon ispresent in an amount of not more than 0.05%, not more than 0.15% copper,and the trace impurities include no more than 0.05% phosphorus and nomore than 0.08% sulfur.